In general, a coating composition is required to have the following properties: a) possession of good melt flow necessary to form a coating film on a surface and a proper curing time at a temperature above 120.degree. C.; b) possession of sufficient glass transition temperature after baking; c) necessity to be solid and brittle in order to permit easy pulverization at processing temperature; d) possession of free fluidity at a temperature below 45.degree. C. without blocking; and e) capacity of being melt mixed with auxiliary substances such as pigment or filler and additives without undergoing premature crosslinking.
A number of coating compositions, have been used which employ alkyd resins, vinyl resins and silicone resins. However, these coating compositions do not necessarily satisfy the required properties. Usually, developing an advantage of one property leads to a deficiency in another property, thereby posing restrictions on the practical use.
Numerous prior art teachings relate to liquid coating composition comprising low molecular weight hydroxy terminated polyesters crosslinked with nitrogen resins or polyisocyanates such as Antonelli, U.S. Pat. No. 4,424,296. Dorffel et al., U.S. Pat. No. 4,101,596 and U.S. Pat. No. 4,204,014 employ curing agents such as aminoplast.
Yapp et al., U.S. Pat. No. 4,169,825 disclose a high solids liquid coating composition comprising low molecular weight hydroxy-terminated polyester, epoxy resin and at least one crosslinking agent such as an isocyanate, an alkoxy alkyl melamine or an amino condensation product.
Other relevant liquid coating compositions include Mysliwczyk, U.S. Pat. No. 5,264,469 which discloses a reaction product of an epoxy resin with carboxyl functional addition polymer, an epoxy phosphate ester polymer and a curing agent such as phenoplast or aminoplast resin.
Patel, U.S. Pat. No. 4,751,256 discloses a heat curable coating composition comprising an aqueous dispersion of epoxy resin, acrylic resins which are copolymers of polymerizable acids and other monomers and alkylated phenol novolac resin.
Scherping et al., U.S. Pat. No. 5,114,993 disclose an aqueous coating composition comprising an epoxy resin, polyester polycarboxylic acid, ethylenically unsaturated monomers containing carboxyl groups, peroxide initiator, crosslinking agent such as phenolic resins and/or amino resins, neutralizing agent, organic solvent and other additives.
Kwon et al., U.S. Pat. No. 5,265,510 disclose a powder coating composition comprising a carboxyl terminated polyester resin and diglycidyl bisphenol epoxy resin which is treated with pyromellitic dianhydride and adipic acid or azelaic acid groups.
Siwiec et al., U.S. Pat. No. 4,092,373 disclose a powder coating composition comprising a mixture of epoxy-functional copolymer of a glycidyl ester of a monoethylenically unsaturated monomer, an anhydride crosslinking agent such as monomeric anhydrides of dicarboxylic acids and homopolymers of monomeric anhydrides of dicarboxylic acids and a hydroxycarboxylic acid.
Arai et al., U.S. Pat. No. 4,459,391 disclose a method of improving the heat resistance of a polyester by reacting a polyester with diglycidyl substituted diamides and keeping the carboxyl group below 15 gram equivalent per 10.sup.6 gram of the polymer.
Guilbert et al., U.S. Pat. No. 4,621,122 disclose a powder coating composition which can withstand a temperature of 200.degree. C. The powder coating composition comprises an acid terminated polyester of an aromatic dicarboxylic acid and a diphenol; an epoxy terminated adduct of a hydantoin diepoxide and an aromatic dicarboxylic acid and an imide dissolved in hydantoin diepoxide; a bismaleimide and a fumaric acid.
Previous coating compositions suffer from disadvantages that include poor color stability at elevated temperature, degradation of gloss with time and temperature, and diminished adhesion after exposure with time at a high temperature. Another disadvantage is that often brittle inflexible coatings are obtained by these types of coating compositions.
There remains a need for a coating composition which overcomes the drawbacks of the prior art. Surprisingly, a resin containing functional hydroxy groups combined with a polyfunctional epoxy resin and an anhydride compound, provides improved adhesion, color/gloss retention, good flow properties, excellent processability, storage stability and low thermoplasticity. The films prepared from the coating compositions are excellent in mechanical, weather, heat and solvent resistance. The composition produces a finish that is able to withstand a prolonged exposure of about 0.degree. C. to about 300.degree. C. The coating has a highly superior adhesion after heating at 300.degree. C. as compared to other silicone and non-silicone containing heat resistant coating composition.